Improve LTMR and GCMR reactor design models

[Reham228]

LTMR

• Replaced UZrH fuel with UO2.
• Number of rings per assembly from 12 to 14
• Reduced enrichment from 19.75% to 5%.
• Increased active height from 78.4 cm to 120 cm.
• Added six internal B4C shutdown rods by replacing moderator positions.
• Updated control drum configuration.
• Increased Monte Carlo statistics for temperature coefficient calculations.

GCMR

• Reduced enrichment from 19.75% to 12%.
• Increased TRISO packing fraction.
• Reduced active height.
• Modified moderator booster radius.
• Added a central shutdown assembly with 12 absorber rods.
• Added six surrounding shutdown assemblies with two absorber rods each.
• Corrected depletion fuel-volume accounting for shutdown assemblies.

Results

The modified designs achieve:

• Positive shutdown margin.
• Negative temperature coefficient.
• Fuel-cycle lifetime greater than three years.